A recently published hydroperoxidase special stain procedure allegedly demonstrated small "phi-body" like cytoplasmic inclusion which were seen almost exclusively in leukocytes from acute myelogenous (AMML). These inclusions appeared in increased numbers and in more cells than the previously well known marker Auer rod. The phi bodies are much smaller than Auer rods. Their increased frequency may be a histochemical marker for disease or early relapse in AML or AMML. The purpose of our current study was to evaluate the usefulness of the hydroperoxidase stain and the relatedness of phi bodies in disease detection.